1. Field of the Invention
The present invention relates to an R-T-B based permanent magnet whose main components are a rare earth element (R), at least one or more kinds of iron element essentially including Fe or Fe and Co (T), and boron (B).
2. Description of the Related Art
R-T-B based permanent magnets have excellent magnetic properties and are thus used for home electric appliances, various kinds of motors such as voice coil motors (VCM) of hard disk drive and motors mounted on hybrid cars, and the like. When the R-T-B based permanent magnet is used for the motor or so, it is required to be excellent in heat resistance for responding to a use environment of high temperature and further have a high coercivity.
As a method for improving coercivity (HcJ) of the R-T-B based permanent magnet, the rare earth element R to which a light rare earth element of Nd, Pr etc. is mainly applied is partially substituted with a heavy rare earth element of Dy, Tb etc. in order to improve crystal magnetic anisotropy of R2T14B phases. It tends to be hard to manufacture a magnet having coercivity large enough to be used for the motors without using the heavy rare earth element.
Dy and Tb, however, are more rare in yield and more expensive than Nd and Pr. In recent years, supply instability of Dy and Tb has been worsening due to rapidly expanding demand in R-T-B based permanent magnets of high coercivity type using a large amount of Dy and Tb. It is thus required to obtain coercivity needed for application to the motors or so even in case of a composition containing Dy and Tb as little as possible.
Under such circumstances, research and development for improving coercivity of R-T-B based permanent magnets without using Dy or Tb have been actively conducted. In the research and development, it is reported that coercivity is improved by a composition using less amount of B than an ordinary R-T-B based permanent magnet.
For example, Patent Document 1 reports that an R-T-B based rare earth sintered magnet using less amount of Dy and having a high coercivity is obtained by having a content of B lower than an ordinary R-T-B based alloy and containing one or more kinds of metal element M selected from Al, Ga, and Cu so as to generate an R2T17 phase, and by sufficiently securing a volume ratio of a transition metal rich phase (R6T13M) generated by using the R2T17 phase as raw material.
Patent Document 2 reports that an R-T-B based sintered magnet having high residual magnetic flux density and coercivity is obtained by having a content of B lower than an ordinary R-T-B based sintered magnet, having a predetermined range of contents of R, B, Al, Cu, Co, Ga, C, and O, and having a ratio of B to Nd and Pr and an atom ratio of Ga and C to B, both of which respectively satisfy a specific relation.
Patent Document 3 reports that an R-T-B sintered magnet having a high Br and a high HcJ is obtained without using Dy by having a composition whose amount of R, amount of B, and amount of Ga are within specific ranges to form a thick two-grain boundary.
Patent Document 1: JP 2013-216965 A
Patent Document 2: WO 2013/191276
Patent Document 3: WO 2014/157448